arm: timer and interrupt init rework

actually the timer init use the interrupt_init as init callback
which make the interrupt and timer implementation difficult to follow

so now rename it as int timer_init(void) and use interrupt_init for interrupt

btw also remane the corresponding file to the functionnality implemented

as ixp arch implement two timer - one based on interrupt - so all the timer
related code is moved to timer.c

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/ixp/Makefile b/cpu/ixp/Makefile
index a673cb1..1403c4f 100644
--- a/cpu/ixp/Makefile
+++ b/cpu/ixp/Makefile
@@ -26,12 +26,10 @@
 LIB	= $(obj)lib$(CPU).a
 
 START	= start.o
+
 COBJS-y	+= cpu.o
-ifndef CONFIG_USE_IRQ
+COBJS-$(CONFIG_USE_IRQ)	+= interrupts.o
 COBJS-y	+= timer.o
-else
-COBJS-y	+= interrupts.o
-endif
 
 SRCS	:= $(START:.o=.S) $(SOBJS-y:.o=.S) $(COBJS-y:.o=.c)
 OBJS	:= $(addprefix $(obj),$(SOBJS-y) $(COBJS-y))